The present invention relates to a gain-controlled amplifier circuit and, more particularly, to a gain control circuit for an amplifier having a stepwise variable gain.
A gain-controlled amplifier ciruit is basically constructed by an amplifier amplifying an input voltage with a variable gain, a detector detecting an amplitude of an output voltage produced by the amplifier and a control circuit comparing the detected amplitude of the output voltage with a reference voltage for controlling the gain of the amplifier in response to the comparison result such that the detected amplitude becomes equal to the reference voltage.
When an amplifier having a linearly variable gain is employed, the above-mentioned basic construction can control the amplifier such that the amplitude of the amplified output voltage becomes stably equal to a voltage corresponding to the reference voltage. However, if an amplifier varying its gain stepwise is employed, the amplified output voltage changes stepwise with a predetermined voltage step. Therefore, unless the detected amplitude of the output voltage is equal to the reference voltage, the gain continues to be changed. That is, the gain of the amplifier is controlled such that the detected amplitude of the output voltage repeats to become larger or smaller than the reference voltage. The amplitude of the amplified output voltage thereby does not become constant.